Super formable high strength steel sheet and method of manufacturing thereof

ABSTRACT

Disclosed herein are a super formable high strength thin steel sheet suitable for use in various applications, e.g., automobiles, and a method for manufacturing the thin steel sheet. The thin steel sheet has a composition which comprises 0.010 wt % or less of C, 0.02 wt % or less of Si, 1.5 wt % or less of Mn, 0.030.15 wt % or less of P, 0.02 wt % or less of S, 0.030.40 wt % of Sol. Al, 0.004 wt % or less of N, 0.0050.040 wt % of Ti, 0.0020.020 wt % of Nb, one or both of 0.0001 0.02 wt % of B and 0.0050.02 wt % of Mo, and the balance of Fe and inevitable impurities, wherein the components P. Mn, Ti, Nb and B satisfy the relationship represented by the following Formulae 1-1 and 1-2, depending on a desired tensile strength: [Formula 1-1]—tensile strength: 35 kg and 40 kg grades 29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.SNb(%)+0.18[B(ppm) or Mo(%)]15=3544.9 [Formula 1-2]—tensile strength: 45 kg grade 29.1+98.3P(%)+4.6Mn(%)−86.STi(%)+62.SNb(%)+0.21 [B(ppm) or Mo(%)]4550, the components Ti, N, C and Nb satisfy the relationship represented by the 2 0 following Formulae 2 and 3 [Formula 2] 0.6 &lt;(1/0.65)(Ti−3.43N)/4C&lt;3.5 [Formnula 3]0.4

TECHNICAL FIELD

[0001] The present invention relates to a super formable high strengththin steel sheet suitable for use in various applications, e.g.,automobiles, and a method for manufacturing the thin steel sheet. Moreparticularly, the present invention relates to a thin steel sheet withexcellent workability and low-temperature annealing properties as aTi—Nb-containing steel in which coarse Ti-based or Nb-based precipitatesare distributed, and a method for manufacturing the thin steel sheet.The thin steel sheet is subjected to surface treatment and has excellentpowdering resistance.

BACKGROUND ART

[0002] In recent years, steel sheets for automobiles tend to be shapedinto an integral body due to their complicated configurations. A highlevel of formability is required to satisfy this tendency. At the sametime, high strength of steel sheets is also required to reduce theweight of car bodies and to ensure safety of drivers. Accordingly,studies on steel sheets having a high strength and high r-value(Lankford value) are thus being actively undertaken.

[0003] Some cold rolled steel sheets for automobiles having a tensilestrength of 35 kgf/mm² grade or more and an r-value of 2.0 or more aredisclosed in (1) Japanese Patent Laid-Open No. 5-230541, (2) U.S. Pat.No. 5,360,493 and (3) Korean Patent Laid-Open No. 2002-0047573.

[0004] (1) According to Japanese Patent Laid-Open No. 5-230541, a steelsheet for automobiles is manufactured by subjecting a steel slab tolubrication hot rolling at a temperature between the Ar₃ transformationpoint and 500° C., and recrystallizing, cold rolling and continuouslyannealing the resulting steel slab, the steel slab comprising aTi—Nb-containing ultra-low carbon steel with 0.2 wt % or less of Al as adeoxidizing element.

[0005] (2) According to U.S. Pat. No. 5,360,493, a steel sheet forautomobiles is manufactured by subjecting a steel slab to lubricationhot rolling at a temperature between the Ar₃ transformation point and500° C., and recrystallizing, cold rolling and continuously annealingthe resulting steel slab, the steel slab comprising a Nb-containing lowcarbon steel with 0.2 wt % or less of Al as an element for precipitatingand fixing AlN.

[0006] Since the prior arts (1) and (2) are, however, techniques inwhich the steel sheets are manufactured by lubrication rolling at theferrite zone, the steel sheets cannot be manufactured by common hotrolling equipments. In addition, the prior arts have disadvantages thatrecrystallization annealing must be carried out before cold rolling andcontinuous annealing temperature is as high as 890° C.

[0007] (3) Korean Patent Laid-Open No. 2002-0047573, which was filed bythe present inventors, relates to a method for manufacturing a coldrolled steel sheet which comprises a Ti—Nb-containing ultra-low carbonsteel with 0.15 wt % or less of Al as a deoxidizing element. The coldrolled steel sheet has a high tensile strength of 40 kgf/mm² grade ormore and a high r-value of 2.0 or more without involvingrecrystallization of a hot rolled sheet, and at the same time, excellentformability. The method lowers the continuous annealing temperature to830° C., but there is a need to further lower it.

[0008] In the prior arts (1), (2) and (3), since a galvanizing orgalvannealing process is applied to the cold rolling steel sheets,powdering resistance of the galvanized layer is an important factor.However, the prior arts fail to mention the powdering resistance.

DISCLOSURE OF THE INVENTION

[0009] Therefore, the present invention has been made in view of theabove problems, and it is an object of the present invention to providea high strength thin steel sheet which can be continuously annealed evenat low temperature and has excellent workability and excellent powderingresistance of a plated layer.

[0010] It is another object of the present invention to provide a methodfor manufacturing the high strength steel sheet.

[0011] In accordance with the present invention, there is provided acold rolled steel sheet having a composition which comprises 0.010 wt %or less of C, 0.02 wt % or less of Si, 1.5 wt % or less of Mn, 0.03˜0.15wt % or less of P, 0.02 wt % or less of S, 0.03˜0.40 wt % of Sol. Al,0.004 wt % or less of N, 0.005˜0.040 wt % of Ti, 0.002˜0.020 wt % of Nb,one or both of 0.0001˜0.02 wt % of B and 0.005˜0.02 wt % of Mo, and thebalance of Fe and inevitable impurities,

[0012] wherein the components P, Mn, Ti, Nb and B satisfy therelationship represented by the following Formulae 1-1 and 1-2,depending on a desired tensile strength:

[0013] Tensile Strength: 35 kg and 40 kg Grades

29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) orMo(%)]=35˜44.9   [Formula 1-1]

[0014] Tensile Strength: 45 kg Grade

29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50,  [Formula 1-2]

[0015] the components Ti, N, C and Nb satisfy the relationshiprepresented by the following Formulae 2 and 3:

0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   [Formula 2]

0.4≦(1/0.35)(Nb/7.75C)≦2.2,   [Formula 3]

[0016] and Ti-based and Nb-based precipitates are distributed in anaverage size ranging from 30˜60 nm.

[0017] In accordance with one aspect of the present invention, there isprovided a galvanized steel sheet having a composition which comprises0.010 wt % or less of C, 0.02 wt % or less of Si, 1.5 wt % or less ofMn, 0.03˜0.15 wt % or less of P, 0.02 wt % or less of S, 0.03˜0.40 wt %of Sol. Al, 0.004 wt % or less of N, 0.005˜0.040 wt % of Ti, 0.002˜0.026wt % of Nb, one or both of 0.0001˜0.02 wt % of B and 0.005˜0.02 wt % ofMo, and the balance of Fe and inevitable impurities,

[0018] wherein the components P, Mn, Ti, Nb and B satisfy therelationship represented by the following Formulae 1-1 and 1-2,depending on a desired tensile strength:

[0019] Tensile Strength: 35 kg and 40 kg Grades

29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) orMo(%)]=35˜44.9   [Formula 1-1]

[0020] Tensile Strength: 45 kg Grade

29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50,  [Formula 1-2]

[0021] the components Ti, N, C and Nb satisfy the relationshiprepresented by the following Formulae 2 and 3:

0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   [Formula 2]

0.4≦(1/0.35)(Nb/7.75C)≦2.2,   [Formula 3]

[0022] Ti-based and Nb-based precipitates are distributed in an averagesize ranging from 30˜60 nm, the steel sheet has a galvanized layerformed on its surface, and the content of Al in the steel sheet is notless than that calculated from the following formula: weight loss in theplated layer=−0.0642Ln (content of sol. Al (%) in the steel)−0.0534.

[0023] In accordance with another aspect of the present invention, thereis provided a method for manufacturing a cold rolled steel sheet,comprising the steps of:

[0024] finish hot rolling a steel slab having a composition of 0.010 wt% or less of C, 0.02 wt % or less of Si, 1.5 wt % or less of Mn,0.03˜0.15 wt % or less of P, 0.02 wt % or less of S, 0.03˜0.40 wt % ofSol. Al, 0.004 wt % or less of N, 0.005˜0.040 wt % of Ti, 0.002˜0.020 wt% of Nb, one or both of 0.0001˜0.02 wt % of B and 0.005˜0.02 wt % of Mo,and the balance of Fe and inevitable impurities at the austenitemonophase zone,

[0025] wherein the components P, Mn, Ti, Nb and B satisfy therelationship represented by the following Formulae 1-1 and 1-2,depending on a desired tensile strength:

[0026] Tensile Strength: 35 kg and 40 kg Grades

29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) orMo(%)]=35˜44.9   [Formula 1-1]

[0027] Tensile Strength: 45 kg Grade

29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50,  [Formula 1-2]

[0028] the components Ti, N, C and Nb satisfy the relationshiprepresented by the following Formulae 2 and 3:

0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   [Formula 2]

0.4≦(1/0.35)(Nb/7.75C)≦2.2;   [Formula 3]

[0029] coiling the resulting steel slab at a temperature meeting thefollowing condition:

730{square root}(1−(Ti*/0.027)²)±15° C. [in which Ti*=Ti(%)−3.43N(%)];

[0030] cold rolling the coil; and

[0031] continuously annealing the cold rolled coil at 780˜830° C.

BRIEF DESCRIPTION THE DRAWINGS

[0032] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0033]FIGS. 1a and 1 b are electron microscope images showing theinfluence of Al content (FIG. 1a: 0.05% (annealing recrystallizationfinish temperature: 830° C., and FIG. 1b: 0.16% (annealingrecrystallization finish temperature: 800° C.)) in a steel on theprecipitation of a cold rolled steel sheet;

[0034]FIG. 2 is a graph showing the influence of Al content in a steelon the r-value of a cold rolled steel sheet;

[0035]FIG. 3 is a graph showing the influence of Al content in a steelon the powdering resistance (weight loss in a galvanized layer) of agalvanized steel sheet;

[0036]FIG. 4 is a graph showing the influence of P, Mn, Ti, Nb and Bcontents on the tensile strength of a cold rolled steel sheet;

[0037]FIG. 5 is a graph showing the influence of Ti, N and C contents onthe r-value of a cold rolled steel sheet;

[0038]FIG. 6 is a graph showing the influence of Nb and C contents onthe r-value of a cold rolled steel sheet; and

[0039]FIG. 7 is a graph showing the influence of coiling temperature onthe r-value of a cold rolled steel sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

[0040] Hereinafter, the present invention will be described in moredetail.

[0041] The thin steel sheet used herein includes cold rolled steelsheets and surface-treated steel sheets such as galvanized steel sheets.The galvanized steel sheets include galvannealed steel sheets. Thetensile strength of 35 kg grade refers to a tensile strength range from35˜39.9 kgf/mm², and the tensile strength of 40 kg grade refers to atensile strength ranging from 40˜44.9 kgf/mm², and the tensile strengthof 45 kg grade refers to a tensile strength ranging from 45˜44.9kgf/mm².

[0042] The present inventors intend to improve the properties of thecold rolled steel sheet disclosed in Korean Patent Laid-Open No.2002-0047573, which was filed by the present inventors. Like other priorarts in the art, Al is used as a deoxidizing element in aTi—Nb-containing steel in Korean Patent Laid-Open No. 2002-0047573 andJapanese Patent Laid-Open No. 5-230541. On the contrary, in U.S. Pat.No. 5,360,493, Al is considered as an element for precipitating andfixing dissolved N.

[0043] The present inventors have paid special attention to novelfunctions of Al which has been considered as a deoxidizing element,particularly, in connection with precipitates, thus accomplishing thepresent invention.

[0044] First, Al contained in a Ti—Nb-containing steel acts as a drivingforce for the formation of coarse Ti-based or Nb-based precipitates,thus significantly increasing the r-value.

[0045] For better workability, the formation of FeTiP precipitates isprevented, and fine Ti-based and Nb-based precipitates (TiC, NbC, TiS,Ti₄C₂S₂) become coarser by a few nm.

[0046] According to the present invention, the Ti-based and Nb-basedprecipitates are coarsely formed to be 30-60 nm in size, thus improvingworkability. Factors affecting the formation of coarse Ti-based andNb-based precipitates and size thereof are Al content and coilingconditions. The addition of Al reduces the distribution of the Ti-basedand Nb-based precipitates and makes the size of the Ti-based andNb-based precipitates coarse. At this time, coiling temperatureconclusively affects the formation of the precipitates. The amount ofeffective Ti (hereinafter, referred to as ‘Ti*’) remaining after bodingwith nitrogen in the steel acts as a driving force for the precipitationof FeTiP or TiC. Accordingly, appropriate control of coiling temperaturedepending on the amount of Ti* can induce the precipitation of TiC,instead of FeTiP. At this time, the size of the TiC precipitates dependson the Al content. FIGS. 1a and 1 b are electron microscope images of alow-Al steel and a high-Al steel. As shown in FIGS. 1a and 1 b, as thedistribution of precipitates in the high-Al steel decreases, the size ofthe precipitates increases. Surprisingly, it was found that the Alcontent and coiling conditions can reduce the distribution of theprecipitates and make the size of the precipitates coarse.

[0047] The effects of the Al content and the coiling conditions on thedistribution of the precipitates and the size thereof in theTi—Nb-containing steel can be determined by the r-value.

[0048] As shown in FIG. 2, the higher the Al content in theTi—Nb-containing steel is, the higher the r-value is. When the Alcontent is not less than 0.151%, particularly 0.21%, the r-value isgreatly improved.

[0049] Second, Al lowers the continuous annealing temperature of theTi—Nb-containing steel.

[0050] P is added to the Ti—Nb-containing steel to increase thestrength, and prevents recrystallization.

[0051] When Al is contained in an amount not less than 0.151%,particularly 0.21%, it impedes the prevention of recrystallization dueto P and promotes the recrystallization, thereby lowering the continuousannealing temperature. In addition, since coarse precipitates aredistributed in the steel of the present invention, annealingrecrystallization delay resulting from fine precipitates can beprevented.

[0052] Third, Al improves the powdering resistance of theTi—Nb-containing steel. It was found that Al diffuses into a surfacelayer along a grain boundary upon plating and makes the plated layercompact, thereby improving the powdering resistance. As shown in FIG. 3,there is a relationship between the Al content and the powderingresistance in the Ti—Nb-containing steel. Based on the relationship,appropriate control of the Al content enables improvement of thepowdering resistance. That is, when the Al content in the steel sheet ishigher than that obtained by the following formula, excellent powderingresistance can be attained: weight loss in the plated layer=−0.0642Ln(content of sol. Al (%) in the steel)−0.0534.

[0053] As described above, the present invention is attributable to thefact that the workability of the Ti—Nb-containing steel can be improvedby the coarse Ti-based or Nb-based precipitates. The reason for limitingthe content range of each component will be explained below.

[0054] [C: 0.01% or Less]

[0055] C contained in the steel is an interstitial dissolved element andprevents the formation of a {111} texture helpful for the workability.Accordingly, it is preferred to limit the content of C in the steel to0.01% or less. As the C content increases, the amount of Ti and Nb,carbonitride-forming elements, increases, which is economicallydisadvantageous. More preferably, the C content is limited to 0.005% orless.

[0056] [Si: 0.02% or Less]

[0057] Si contained in the steel causes scale defects on the surface,and generates a temper color upon annealing and non-plated regions uponplating. Accordingly, it is preferred to limit the content of Si in thesteel to 0.02% or less.

[0058] [Mn: 1.5% or Less]

[0059] Mn contained in the steel is a substitutional solid solutionstrengthening element, and is added for strength improvement. When theMn content exceeds 1.5%, elongation and r-value are drasticallydecreased. Accordingly, it is preferred to limit the content of Mn inthe steel to 1.5% or less.

[0060] [0.03˜0.15%]

[0061] Like Mn, P contained in the steel is a solid solutionstrengthening element. P increases the strength of the Ti—Nb-based steelgrades of the steel of the present invention, and develops a {111}texture helpful to increase the r-value due to fine graining andboundary segregation, etc. When the P content exceeds 0.15%, elongationis considerably reduced and the embrittlement of the steel is greatlyincreased. Accordingly, it is preferred to limit the content of P in thesteel to 0.03%-0.15%.

[0062] [S: 0.02% or Less]

[0063] As the S content is further lowered, it is more advantageous interms of the workability of the steel sheet. Accordingly, the S contentis commonly maintained at a level of 0.005% or lower. Since Mn in thesteel is bonded to S to form MnS, the deterioration of workability dueto dissolved S can be avoided. Accordingly, it is preferred to limit thecontent of P in the steel to 0.02% or less in which the occurrence ofedge cracks can be avoided.

[0064] [Sol. Al: 0.03˜0.40%]

[0065] Sol. Al is the most important element in the present invention,and impedes the prevention of recrystallization due to P, therebypromoting recrystallization. Sol. Al diffuses into a surface layer alonga grain boundary upon plating and makes the plated layer compact,thereby improving the powdering resistance. The addition of Al reducesthe distribution of the Ti-based and Nb-based precipitates (TiC, NbC,TiS, Ti₄C₂S₂) and makes the size of the Ti-based and Nb-basedprecipitates coarse, thereby increasing the r-value. These functions ofSol. Al are possible only when the Sol. Al content is 0.03% or more,preferably 0.151% or more, and more preferably 0.21% or more. When theSol. Al content is higher than 0.4%, considerable cost is taken andoperating efficiency for continuous casting is deteriorated.

[0066] [N: 0.004% or Less]

[0067] Too high N content causes deteriorated workability. As the Ncontent increases, the Ti content is undesirably increased. Accordingly,it is preferred to limit the content of N in the steel to 0.004% orless, if possible.

[0068] [Ti: 0.005˜0.040%, Nb: 0.002˜0.020%]

[0069] Ti and Nb are important elements in terms of workability(particularly, r-value). For improved workability, Ti and Nb arepreferably added in an amount of 0.005% or more and 0.002% or more,respectively. The Ti content and the Nb content exceeding 0.040% and0.020%, respectively, are economically disadvantageous. Accordingly, itis preferred to limit the content of Ti and Nb to 0.005˜0.04% and0.002˜0.020%, respectively.

[0070] [One or Both of 0.0001˜0.02 wt % of B and 0.005˜0.02 wt % of Mo]

[0071] B and Mo contained in the steel are elements useful forpreventing P from embrittling the grain boundaries and prevent a secondworking embrittlement. If a mixture of B and Mo is added, there is arisk of low r-value and increased cost.

[0072] Accordingly, one element selected from B and Mo is preferablyadded. Considering that exact control of the amount of B is difficult,the addition of Mo is more preferable.

[0073] In the present invention, the amounts of B and Mo added for asecond working embrittlement are 0.0001% or more and 0.005% or more,respectively. When the amounts of B or Mo added are more.than 0.002% and0.02%, respectively, workability is considerably reduced.

[0074] In order to attain a desired strength and a high r-value of theTi—Nb-containing steel according to the present invention, theTi—Nb-containing steel must meet the following formulae 1 to 3.

[0075] Formulae 1-1 and 1-2 are equations which are regressivelyobtained from empirical equations expressed by numerically representingthe influence of each component on the tensile strength. Formulae 1-1and 1-2 are based on the fact that Ti and Nb other than P, Mn and B mayaffect the strength of the steel. Ti promotes the precipitation of FeTiPand thus reduces the strengthening effect of P, a solid solutionstrengthening element. In addition, Nb is self-dissolved and thusincreases the strength of the steel.

[0076] The elements P, Mn, Ti, Nb and B are preferably added so as tosatisfy the relationship represented by the following formula 1-1 or 1-2depending on a desired strength. Formula 1-1 is applied to 35 kg and 40kg grades, and Formula 1-2 is applied to a 45 kg grade.

29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18(B(ppm) orMo(%))=35˜44.9   [Formula 1-1]

29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21(B or Mo)(ppm)=45˜50  [Formula 1-2]

[0077] As can be seen from FIG. 4, values (tensile strength) calculatedby Formulae 1-1 and 1-2 depending on the contents of P, Mn, Ti, Nb and Bare substantially coincident with measured values. Accordingly, thepresent invention has an advantage in that a desired grade (tensilestrength) of a cold rolled steel sheet can be freely designed within therange of 35˜50 kg/mm². In FIG. 4, 35 kg and 40 kg grades are given byFormula 1-1, and a 45 kg grade is given by Formula 1-2.

[0078] When the contents of Ti and Nb, carbonitride-forming elements, inthe Ti—Nb-containing steel satisfy the relationship represented by thefollowing formulae 2 and 3, workability can be improved. That is, as canbe seen from FIGS. 5 and 6, r-values are dependent on Formulae 2 and 3below:

0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   [Formula 2]

0.4≦(1/0.35)(Nb/7.75C)≦2.2   [Formula 3]

[0079] Formula 2 defines the amount of Ti added. When the atomicequivalence ratio between 65% [=(1/0.65)(Ti−3.43N)] of the amountremaining after Ti equivalently bonds with dissolved N, and dissolvedcarbon in the steel is less than 0.6, the fixation of the dissolvedcarbon is unstable and the r-value is decreased. When the atomicequivalence ratio exceeds 3.5, the remaining amount of Ti is too largeand thus a large amount of FeTiP precipitates is formed, decreasing ther-value. Formula 2 preferably optimizes the amount of Ti added forimproved workability. An experimental result demonstrates that 65% ofthe amount remaining after Ti equivalently bonds with dissolved N bondswith dissolved C. That is, since most of the carbon precipitates are inthe form of (Ti, Nb)C, the measurement of the content ratio of Ti to Nb,which participate in the fixation of the dissolved carbon, demonstratesthat the ratio is 65%:35%.

[0080] In addition, Formula 3 defines the amount of Nb added. When theratio of the Nb content in the steel to dissolved carbon is less then0.4, incomplete scavenging may be increased. When the ratio exceeds 2.2,the amount of dissolved Nb in the steel increases, causing poorworkability. Accordingly, the amount of Nb added for excellentworkability is preferably optimized by the Formulae expressed above.

[0081] The Ti-based and Nb-based precipitates are distributed in anaverage size ranging from 30˜60 nm in the Ti—Nb-containing steel of thepresent invention. When the average size of the precipitates is smallerthan 30 nm, workability is poor. The coarser the precipitates are, thebetter the workability is. However, when the average size of theprecipitates is larger than 60nm, the amount of FeTiP adverselyaffecting the workability is undesirably increased. That is, in order toobtain precipitates having a size of 60 nm or larger, high coilingtemperature is required. It was identified in the present invention thatincrease of coiling temperature leads to more FeTiP precipitates.Accordingly, the upper limit of the size of the coarse precipitatescapable of preventing the precipitation of FeTiP was proved to be 60 nm.

[0082] A galvanized layer is formed on the surface of the cold rolledsteel sheet according to the present invention. At this time, the Alcontent in the cold rolled steel sheet influences the powderingresistance of the galvanized layer. The following formula isregressively obtained from the relationship between the weight loss inthe plated layer (upon powdering evaluation) and the content of Al inthe steel sheet: Weight loss in the plated layer=−0.0642Ln (content ofsol. Al (%) in the steel)−0.0534.

[0083] A galvanized steel sheet having a weight loss in a plated layerless than a reference can be manufactured in accordance with thefollowing procedure: After a reference weight loss in a plated layer isdetermined, it is applied to the formula described above to calculatethe Al content in the steel sheet. Next, Al is added in an amount higherthan the calculated Al content to manufacture a galvanized steel sheethaving a weight loss less than the reference.

[0084] Next, the method of the present invention will be explained.

[0085] [Hot rolling process]

[0086] The steel slab thus manufactured is reheated, and then hot rolledunder finish rolling conditions at an Ar₃ transformation point. The Ar₃transformation point in the Ti—Nb-containing steel of the presentinvention is about 900° C. When the finish rolling temperature is in adiphase zone at a temperature not higher than the Ar₃ transformationpoint, a texture adversely affecting the r-value is undesirablydeveloped.

[0087] Subsequently, the hot rolled steel sheet is coiled.

[0088] The coiling temperature (CT) must meet the following Formula 4:

CT=730{square root}(1−(Ti*/0.027)2)±15° C.   [Formula 4]

[0089] wherein, Ti* represents Ti(%)−3.43N(%).

[0090] Ti* refers to the amount of effective Ti remaining after bodingwith nitrogen in the steel. Accordingly, in the case that the amount ofeffective Ti is relatively large, there is a large possibility thatFeTiP adversely affecting the workability may be precipitated. Toprevent the precipitation of FeTiP, low temperature coiling ispreferably carried out. In the case that the amount of effective Ti isrelatively small, the fixation of dissolved carbon into the form of TiCprecipitates is required to attain a high r-value. For this purpose,high temperature coiling is preferably carried out. Formula 4 is anempirical expression obtained in view of the driving force for theformation of coarse precipitates depending on the amount of effectiveTi.

[0091] As can be seen from FIG. 7, the coiling temperature is dependenton Formula 4. As shown in FIG. 7, the r-value is good within the rangeof the coiling temperature calculated by Formula 4±15° C.

[0092] [Cold Rolling Process]

[0093] The hot rolled steel sheet thus coiled is cold rolled.

[0094] To attain a high revalue, the cold rolling is preferably carriedout at a cold rolling reduction rate of 70% or more. More preferably,the cold rolling is carried out at a cold rolling reduction rate of70˜90%.

[0095] [Continuous Annealing Process]

[0096] The cold rolled steel sheet thus cold rolled is annealed.

[0097] The annealing is preferably continuously carried out. Theannealing temperature is preferably within the range of 780˜860° C. Whenthe annealing temperature is lower than 780° C., it is almost impossibleto obtain an r-value of 2.0 or more. When the annealing temperature ishigher than 860° C., there may be a problem in the shape of a strip dueto high temperature annealing during processing. When the Al content inthe Ti—Nb-containing steel of the present invention is not lower than0.151% or 0.21%, the annealing temperature can be lowered to 830° C. orless. The annealing temperature is preferably carried out at 780˜830° C.

[0098] After the continuous annealing, cooling is preferably carried outat a rate of 7˜30° C./sec. For example, the cooling rate is preferably15˜30° C./sec in the case of a steel sheet having a tensile strength of45 kg grade. When the cooling rate is less than 15° C./sec, it isdifficult to obtain a tensile strength of 45 kg grade.

[0099] After the continuous annealing, skin pass rolling may be carriedout at an appropriate reduction rate for controlling the shape orsurface roughness. In addition, the cold rolled steel sheet of thepresent invention can be applied to original sheets of surface-treatedsteel sheets. Examples of the surface-treatment include galvanizing andgalvannealing, etc. Galvanizing and, if necessary, galvannealing may becarried out following the continuous annealing.

[0100] Hereinafter, the present invention will be described in moredetail with reference to the following Examples.

[0101] Formulae 1 to 4 shown in the Tables below are as follows:

[0102] Tensile Strength: 35 kg and 40 kg Grades

29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) orMo(%)]=35˜44.9   [Formula 1-1]

[0103] Tensile Strength: 45 kg Grade

29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50,  [Formula 1-2]

0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   [Formula 2]

0.4≦(1/0.35)(Nb/7.75C)≦2.2   [Formula 3]

730{square root}(1−(Ti*/0.027)²)±15° C. [in whichTi*=Ti(%)−3.43N(%)]  [Formula 4]

EXAMPLE 1

[0104] After a steel slab shown in Table 1 below was hot rolled above anAr₃ transformation point and coiled, the resulting coil was cold rolledand continuously annealed under the conditions shown in Table 2 below tomanufacture a cold rolled steel sheet. The mechanical properties of thecold rolled steel sheet are shown in Table 2 below. As shown in Table 1,the content of both Si and S was 0.01%. TABLE 1 Chemical components (wt%) Values calculated Steel B from Formulae No. C Mn P S.Al N Ti Nb (ppm)Formula 1 Formula 2 Formula 3 Remarks 1 0.0027 0.5 0.04 0.05 0.00180.015 0.011 5 35.3 1.3 1.5 Tensile strength 2 0.0026 0.58 0.039 0.210.0027 0.017 0.01 7 35.4 1.1 1.4 35 kg grade 3 0.0032 0.6 0.042 0.040.0017 0.02 0.013 3 35.1 1.7 1.5 (Formula 1-1 4 0.0029 0.53 0.042 0.300.0023 0.016 0.006 9 35.3 1.1 0.8 application) 5 0.0038 0.48 0.061 0.030.0021 0.045 — 8 — — — 6 0.0031 0.38 0.058 0.04 0.0029 0.048 — 5 — — — 70.0027 0.880 0.110 0.06 0.0025 0.024 0.007 9 41.9 2.2 1.0 Tensilestrength 8 0.0021 1.020 0.091 0.04 0.0023 0.016 0.010 10 42.4 1.5 1.8 40kg grade 9 0.0031 0.780 0.102 0.17 0.002 0.021 0.013 8 41.9 1.8 1.5(Formula 1-1 10 0.0025 1.150 0.087 0.24 0.0026 0.018 0.006 12 42.1 1.40.9 application) 11 0.0038 0.830 0.095 0.04 0.0028 0.043 — 7 — — — 120.0033 0.950 0.105 0.03 0.0022 0.049 — 5 — — — 13 0.0026 1.12 0.096 0.040.0026 0.016 0.007 8 45.1 1.0 1.0 Tensile strength 14 0.0031 1.09 0.0940.05 0.0021 0.017 0.008 11 45.5 1.2 1.0 45 kg grade 15 0.0027 1.18 0.0890.17 0.0028 0.019 0.006 12 45.1 1.3 0.8 (Formula 1-2 16 0.0034 1.250.104 0.34 0.0031 0.023 0.01 7 46.2 1.4 1.1 application) 17 0.0039 1.210.093 0.04 0.0025 0.052 — 6 — — — 18 0.0032 1.24 0.095 0.05 0.0029 0.049— 9 — — —

[0105] TABLE 2 Cold rolling Continuous Powdering reduction annealingTensile resistance Steel rate temperature strength Elongation (Weightloss in No. (%) (° C.) (kg/mm²) (%) r-value plated layer) Remarks 1 73843 35.2 43.2 2.34 12% Tensile strength 2 75 804 35.9 44.1 2.41 6% 35 kggrade 3 75 836 36.1 45.0 2.28 10% 4 73 795 36.8 44.3 2.45 5% 5 75 83035.8 45.2 1.89 18% 6 75 830 35.4 45.3 1.85 14% 7 75 835 42.1 35.9 2.218% Tensile strength 8 77.5 841 41.9 36.2 2.18 9% 40 kg grade 9 75 79641.6 37.0 2.26 4% 10 77.5 812 42.1 36.7 2.41 3% 11 75 830 41.2 37.2 1.829% 12 73 830 40.9 36.8 1.79 19% 13 75 843 45.5 33.9 2.18 11% Tensilestrength 14 77.5 841 46.3 33.2 2.13 13% 45 kg grade 15 75 803 46.6 34.02.26 6% 16 77.5 815 47.1 33.7 2.34 4% 17 75 840 45.2 34.2 1.78 12% 18 73830 45.9 33.8 1.75 20%

[0106] The r-values shown in Table 2 were measured by imparting atensile pre-strain of 15%, and then averaging the values obtained at theL-direction (rolling direction), the D-direction (45° to the rollingdirection) and the C-direction (90° to the rolling direction) asfollows: r=(rL+2rD+rC)/4 in accordance with the three-point method. Inaddition, powdering resistance, that is, the weight loss in a platedlayer was obtained by punching out a test piece in a disk having aradius of 100 mm, cupping at an elongation of 2.0 and weighing.

[0107] As shown in Tables 1 and 2, the steel sheet of the presentinvention can be freely designed into 35 kg, 40 kg, 45 kg grades, etc.In addition, the steel sheet of the present invention can have anr-value of 2.0 or more. Furthermore, upon powdering evaluation, theweight loss in a plated layer can be considerably reduced.

EXAMPLE 2

[0108] After a steel slab shown in Table 3 below was hot rolled above anAr₃ transformation point and coiled, the resulting coil was cold rolledat a cold rolling reduction rate of 77% and continuously annealed at830° C. to manufacture a cold rolled steel sheet. The mechanicalproperties of the cold rolled steel sheet are shown in Table 4 below. Asshown in Table 3, the content of both Si and S was 0.01%. TABLE 3 Hotrolling Values measured condition Steel Chemical components (wt %) fromformulae (° C.) No. C Mn P S.Al N Ti Nb Mo B (ppm) Form. 1 Form. 2 Form.3 FDT CT 19 0.0031 0.98 0.11 0.05 0.0025 0.024 0.007 0.007 — 40.6 1.90.8 913 587 20 0.0024 1.01 0.091 0.18 0.0023 0.016 0.01 0.012 — 40.6 1.31.5 910 638 21 0.0028 0.89 0.102 0.08 0.002 0.021 0.008 0.016 — 40.1 1.91.1 908 599 22 0.0025 1.05 0.095 0.23 0.0026 0.018 0.007 — — 40.4 1.41.0 911 628 23 0.0038 0.930 0.095 0.05 0.0028 0.043 0.005 — 8 — — — 905630 24 0.0033 0.950 0.105 0.04 0.0022 0.049 0.007 — 5 — — — 900 610

[0109] TABLE 4 Ductile-brittle Tensile properties Steel transitionTensile strength No. temperature (° C.) (kg/mm²) Elongation (%) r-values19 −40 41.1 35.0 2.17 20 −45 41.8 36.1 2.18 21 −40 41.0 36.8 2.09 22 542.1 36.7 2.18 23 −40 41.2 37.6 2.06 24 −45 40.9 36.9 2.08

EXAMPLE 3

[0110] A steel slab shown in Table 5 below was finish hot rolled at 910°C. to obtain a hot rolled steel sheet. After the hot rolled steel sheetwas coiled under the conditions shown in Table 6, the resulting coil wascold rolled at a cold rolling reduction rate of 77% and continuouslyannealed under the conditions shown in Table 7 below. The mechanicalproperties of the cold rolled steel sheet are shown in Table 6 below.

[0111] As shown in Table 5, the content of both Si and S was 0.01%.TABLE 5 Values calculated Steel Chemical components (wt %) from FormulaeNo. C Mn P S.Al N Ti Nb B (ppm) Ti* Form. 1 Form. 2 Form. 3 25 0.00250.92 0.11 0.05 0.0025 0.024 0.007 9 0.015 41.7 2.2 1 26 0.0031 1.010.096 0.06 0.0023 0.016 0.01 10 0.008 42.3 1.5 1.8 27 0.0022 0.78 0.1040.07 0.002 0.021 0.013 8 0.014 41.9 1.8 1.5 28 0.0027 1.12 0.087 0.050.0026 0.018 0.006 12 0.009 42 1.4 0.9 29 0.0038 0.83 0.095 0.03 0.00280.043 — 7 — — — 30 0.0033 0.95 0.105 0.04 0.0022 0.049 — 5 — — — 310.0031 1.12 0.092 0.08 0.0024 0.017 0.008 — 0.009 40.7 1.1 1.0 (0.012%Mo)

[0112] TABLE 6 Target CT Coiling calculated temperature AnnealingTensile Steel from (Measured Temperature strength No. Formula 4 value)(° C.) (kg/mm²) r-values 25 599 ± 15° C. 595 845 41.5 2.28 26 696 ± 15°C. 690 840 41.3 2.33 27 621 ± 15° C. 620 845 40.6 2.26 28 688 ± 15° C.680 850 42.1 2.31 28 688 ± 15° C. 600 850 42.3 1.92 29 — 630 830 41.21.78 30 — 630 830 42.2 1.75 31 688 ± 15° C. 685 838 41.1 2.26

730{square root}(1−(Ti*/0.027)²   Formula 4

[0113] As can be seen from Table 6, if a steel sheet is manufactured bycoiling the steel manufactured in accordance with the method of thepresent invention at a coiling temperature (target temperature) obtaineddepending on the effective amount of Ti*, super formable and highstrength steels having a very high r-value can be stably manufactured.

EXAMPLE 4

[0114] A steel slab shown in Table 7 below was finish hot rolled at 910°C. to obtain a hot rolled steel sheet having a thickness of 3.2 mm.After the hot rolled steel sheet was coiled under the conditions shownin Table 8, the resulting coil was cold rolled at a cold rollingreduction rate of 77%. The annealing recrystallization finishtemperature and the mechanical properties of the cold rolled steel sheetwere measured the results are shown in Table 8 below.

[0115] As shown in Table 7, the content of both Si and S was 0.01%.TABLE 7 Values calculated Steel Chemical components (wt %) from No. C MnP N S.Al Ti Nb B (ppm) Ti* Formula 4 32 0.0027 0.88 0.11 0.0025 0.050.024 0.007 9 0.015 607 ± 15° C. 33 0.0031 0.78 0.102 0.002 0.27 0.0210.013 8 0.014 624 ± 15° C.

[0116] TABLE 8 Target Measured Size of Annealing Steel coiling coilingprecipitates Other properties recrystallization No. temperatureTemperature (mean, nm) of precipitates r-values finish temperature 32607 ± 15° C. 550 15 Precipitates having 1.86 830 a size of 10 nm or lesswere observed 610 37 2.36 820 680 56 A large amount of 1.98 820 FeTiPwas observed 33 624 ± 15° C. 550 23 1.96 810 620 42 2.43 790 700 62 Alarge amount of 2.05 790 FeTiP was observed

[0117] As shown in Table 8, when the sheet was coiled at a lowtemperature relative to the target coiling temperature, ultrafineprecipitates were observed. The presence of the ultrafine precipitateslowered the r-value and increased the annealing recrystallization finishtemperature. Too high coiling temperature resulted in the formation of alarge amount of FeTiP in the steel, a cause of low revalue. FeTiP wasdecomposed during annealing, and impeded the development of therecrystallized texture. When the S. Al content was high as in steel No.33, precipitates were stably. formed (slightly increased in size) andthus the workability was improved and the annealing recrystallizationtemperature was lowered.

[0118] Industrial Applicability

[0119] As apparent from the above description, the thin steel sheetaccording to the present invention exhibits excellent workability,low-temperature annealing properties and excellent powdering resistanceby reduced distribution of the Ti-based precipitates, etc. and coarsesize of the precipitates.

[0120] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A super formable high strength thin steel sheet having a compositionwhich comprises 0.010 wt % or less of C, 0.02 wt % or less of Si, 1.5 wt% or less of Mn, 0.03˜0.15 wt % or less of P, 0.02 wt % or less of S,0.03˜0.40 wt % of Sol. Al, 0.004 wt % or less of N, 0.005˜0.040 wt % ofTi, 0.002˜0.020 wt % of Nb, one or both of 0.0001˜0.02 wt % of B and0.005˜0.02 wt % of Mo, and the balance of Fe and inevitable impurities,wherein the components P, Mn, Ti, Nb and B satisfy the relationshiprepresented by the following Formulae 1-1 and 1-2, depending on adesired tensile strength: at tensile strength of 35 kg and 40 kg grades29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) orMo(%)]=35˜44.9   Formula 1-1 and at tensile strength of 45 kg grade29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50  Formula 1-2, wherein the components Ti, N, C and Nb satisfy therelationship represented by the following Formulae 2 and 3:0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   Formula 2 0.4≦(1/0.35)(Nb/7.75C)≦2.2  Formula 3, and Ti-based and Nb-based precipitates are distributed inan average size ranging from 30˜60 nm.
 2. A super formable high strengththin steel sheet with excellent powdering resistance, the thin steelsheet having a composition which comprises 0.010 wt % or less of C, 0.02wt % or less of Si, 1.5 wt % or less of Mn, 0.03˜0.15 wt % or less of P,0.02 wt % or less of S, 0.03˜0.40 wt % of Sol. Al, 0.004 wt % or less ofN, 0.005˜0.040 wt % of Ti, 0.002˜0.020 wt % of Nb, one or both of0.0001˜0.02 wt % of B and 0.005˜0.02 wt % of Mo, and the balance of Feand inevitable impurities, wherein the components P, Mn, Ti, Nb and Bsatisfy the relationship represented by the following Formulae 1-1 and1-2, depending on a desired tensile strength: at tensile strength of 35kg and 40 kg grades29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) orMo(%)]=35˜44.9   Formula 1-1, and at tensile strength of 45 kg grade29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50  Formula 1-2, and wherein the components Ti, N, C and Nb satisfy therelationship presented by the following Formulae 2 and 3:0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   Formula 2, and0.4≦(1/0.35)(Nb/7.75C)≦2.2   Formula 3, and wherein Ti-based andNb-based precipitates are distributed in an average size ranging from30˜60 nm, the steel sheet has a galvanized layer formed on its surface,and the content of Al in the steel sheet is not less than thatcalculated from the following formula: weight loss in the platedlayer=−0.0642Ln (content of Sol. Al (%) in the steel)−0.0534.
 3. Thesuper formable high strength thin steel sheet according to claim 1,wherein the Al content is 0.151˜0.4%.
 4. The super formable highstrength thin steel sheet according to claim 1, wherein the Al contentis 0.21˜0.4%.
 5. A method for manufacturing a super formable highstrength thin steel sheet, comprising the steps of: finish hot rolling asteel slab having a composition of 0.010 wt % or less of C, 0.02 wt % orless of Si, 1.5 wt % or less of Mn, 0.03˜0.15 wt % or less of P, 0.02 wt% or less of S, 0.03˜0.40 wt % of Sol. Al, 0.004 wt % or less of N,0.005˜0.040 wt % of Ti, 0.002˜0.020 wt % of Nb, one or both of0.0001˜0.02 wt % of B and 0.005˜0.02 wt % of Mo, and the balance Fe andinevitable impurities at the austenite monophase zone, wherein thecomponents P, Mn, Ti, Nb and B satisfy the relationship represented bythe following Formulae 1-1 and 1-2, depending on a desired tensilestrength: at tensile strength of 35 kg and 40 kg grades29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) orMo(%)]=35˜44.9   Formula 1-1, and at tensile strength of 45 kg grade29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50  Formula 1-2, and wherein the components Ti, N, C and Nb satisfy therelationship represented by the following Formulae 2 and 3:0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   Formula 2, and0.4≦(1/0.35)(Nb/7.75C)≦2.2   Formula 3, and coiling the resulting steelslab at a temperature meeting the following condition: 730 {squareroot}(1−(Ti*0.027)²)±15° C. [in which Ti*=Ti(%)−3.43N(%)]; cold rollingthe coil; and continuously annealing the cold rolled coil at 780˜860° C.6. The method for manufacturing a super formable high strength thinsteel sheet according to claim 5, wherein the Al content is 0.151˜0.4%.7. The method for manufacturing a super formable high strength thinsteel sheet according to claim 5, wherein the Al content is 0.21˜0.4%.8. The method for manufacturing a super formable high strength thinsteel sheet according to claim 1, wherein the continuous annealing iscarried out at 780˜830° C.
 9. The method for manufacturing a superformable high strength thin steel sheet according to claim 2, whereinthe continuous annealing is carried out at 780˜830° C.
 10. The superformable high strength thin steel sheet according to claim 2, whereinthe Al content is 0.151˜0.4%.
 11. The super formable high strength thinsteel sheet according to claim 2, wherein the Al content is 0.21˜0.4%.